/**
 * Created with IntelliJ IDEA.
 * Description:
 * User: 26727
 * Date: 2024-08-14
 * Time: 23:21
 */
class Solution6 {
    int[] tmp;
    public int reversePairs(int[] record) {
        int n = record.length;
        tmp = new int[n];
        return mergeSort(record,0,n-1);
    }

    public int mergeSort(int[] nums,int left,int right) {
        if(left >= right) {
            return 0;
        }

        int mid = (left+right)/2;
        int ret = 0;
        ret += mergeSort(nums,left,mid);
        ret += mergeSort(nums,mid+1,right);

        int cur1 = left,cur2 = mid+1,i=0;
        while(cur1 <= mid && cur2 <= right) {
            if(nums[cur1] <= nums[cur2]) {
                tmp[i++] = nums[cur1++];
            }else {
                ret += mid - cur1 + 1;
                tmp[i++] = nums[cur2++];
            }
        }

        while(cur1 <= mid) {
            tmp[i++] = nums[cur1++];
        }
        while(cur2 <= right) {
            tmp[i++] = nums[cur2++];
        }

        for(int j = left; j <= right; j++) {
            nums[j] = tmp[j-left];
        }
        return ret;
    }
}